# Wireless & Mobile Communications Questions & Answers – Combined Linear and Constant Envelope Modulation Techniques

This set of Wireless & Mobile Communications Quiz focuses on “Combined Linear and Constant Envelope Modulation Techniques”.

1. Which of the following is a combined linear and constant envelope technique?
a) MPSK
b) PSK
c) BPSK
d) QPSK

Explanation: M-ary phase shift keying (MPSK) is a combined linear and constant envelope technique. It is a part of M-ary modulation techniques. These modern modulation techniques exploit the fact that digital baseband data may be sent by varying both the envelope and phase of an RF carrier.

2. In an M-ary signalling scheme two or more bits are grouped together to form a _______
a) Chip
b) Symbol
c) Byte
d) Pattern

Explanation: In an M-ary signalling scheme two or more bits are grouped together to form symbols. And one of the M possible signals is transmitted during each symbol period of duration Ts.

3. The number of possible signal in M-ary signalling is given by M and M = __________ where n is an integer.
a) n
b) 2n
c) 2n
d) n2

Explanation: Two or more bits are grouped to form a symbol in M-ary modulation. And the number of possible symbols should be equal to 2n, where n is an integer.

4. M-ary signalling techniques are not sensitive to timing jitters.
a) True
b) False

Explanation: Timing errors increase when smaller distances between signals in the constellation diagram are used. M-ary signalling techniques are attractive for use in bandlimited channel, but are limited in their applications due to sensitivity in timing jitter.

5. M-ary modulation schemes have very good power efficiency.
a) True
b) False

Explanation: M-ary modulation schemes have poor power efficiency, but they have a better bandwidth efficiency. An 8-PSK system requires a bandwidth that is 3 times smaller than a BPSK system, whereas its BER performance is very worse since signals are packed more closely in the signal constellation.
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6. In M-ary PSK, the carrier ___________ takes one of M possible values.
a) Amplitude
b) Frequency
c) Angle
d) Phase

Explanation: In an M-ary PSK, the carrier phase takes one of the M possible values. The possible values of phase are θi=2(i-1)/M, where i=1,2,…….,M.

7. The constellation of M-ary PSK is ____________ dimensional.
a) One
b) Does not exist
c) Two
d) Three

Explanation: The constellation of M-ary PSK is two dimensional. It is due to the presence of two basis signals. And the M-ary message points are equally spaced on a circle.

8. What is the radius of the circle in M-ary PSK on which message points are equaly spaced?
a) √Es
b) √Eb
c) Eb
d) Es

Explanation: The M-ary message points are equally spaced on a circle of radius √Es centred at the origin. Here Es is energy per symbol. Thus, MPSK is a constant envelope signal when no pulse shaping is used.

9. As the value of M _________ the bandwidth efficiency ________
a) Increases, same.
b) Increases, decreases
c) Increases, increases
d) Decreases, same

Explanation: The first null bandwidth of M-ary PSK signals decrease as M increases while Rb is held constant. Therefore, as the value of M increases, the bandwidth efficiency also increases.

10. The power efficiency of the M ary PSK decreases because of the _____
a) Freely packed constellation
b) Increment of bandwidth efficiency
c) Fixed null bandwidth
d) Densely packed constellation

Explanation: Bandwidth efficiency increases as the value of M increases. But at the same time, increasing M implies that the constellation is more densely packed. Hence the power efficiency or noise tolerance is decreased.

11. In QAM, the amplitude is _______ and phase is _______
a) Varied, constant
b) Varied, varied
c) Constant, varied
d) Constant, constant

Explanation: Quadrature amplitude modulation (QAM) is obtained by allowing the amplitude to also vary with the phase. Thus, the constellation consists of square lattice of signal points.

12. M-ary QAM signal have constant energy per symbol.
a) True
b) False

Explanation: M-ary QAM does not have constant energy per symbol. It also does not have constant distance between possible symbol states. It reasons that particular values of M-ary QAM signal will be detected with higher probability than others.

13. In comparison to M-ary PSK, M-ary QAM bandwidth efficiency is _____ and power efficiency is ________
a) Identical, superior
b) Less, superior
c) Identical, identical
d) Superior, superior

Explanation: The power spectrum and bandwidth efficiency of QAM modulator is identical to M-ary PSK modulation. But, in terms of power efficiency QAM is superior to M-ary PSK.

14. The bandwidth efficiency of an M-ary FSK signal ________ with ________ in M.
a) Constant, increase
b) Increases, increase
c) Decreases, increase
d) Decreases, decrease

Explanation: The bandwidth efficiency of an M-ary FSK signal decreases with increase in M. Therefore, unlike M-PSK signals, M-FSK signals are bandwidth inefficient.

15. Power efficiency of M-ary FSK increases, since _________
a) Constellation is densely packed
b) M signals are non-orthogonal
c) Fixed null bandwidth
d) M-signals are orthogonal

Explanation: In M-ary FSK, all the M signals are orthogonal and there is no crowding in the signal space. Hence, power efficiency of M-ary FSK increases with M.

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